There are many facilities which require that energy, such as that necessary to provide heat, refrigeration or simply electrical power, be added and/or subtracted incrementally. For instance, in an institution, or a large office building, having a substantial number of window air conditioners, if all of the air conditioners were turned on at the same time, there would be a requirement that the electrical energy be supplied incrementally. If this were not the case, the instantaneous electrical current demand would be extremely high and if not controlled in some way, would cause circuit breakers to be tripped and/or equipment to be damaged.
It has been determined that it is preferable to add an increment of power, then allow the surge condition to settle, and thereafter add another increment of power until such time as the full demand has been met. These problems are also present when power is turned off and therefore it is desirable to turn off power by increments. My two patents numbered U.S. Pat. Nos. 3,730,819 and 3,717,300 describe systems for adding and subtracting power units in heating and cooling systems. While these patented systems have been quite satisfactory, there has been a need for a more flexible controller system which can operate with any one of a plurality of user's monitoring signals and which can operate either in a "first-on, first-off" mode of operation, or a "first-on, last-off" mode of operation. The present system provides this flexibility.